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question on connecting pc power supply in series

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  • urstest
    I have been using some old pc power supply for my projects. But currently I am working on a new project that needs about 24v to drive stepper motors. I am
    Message 1 of 2 , Mar 18, 2002
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      I have been using some old pc power supply for my projects. But
      currently I am working on a new project that needs about 24v to drive
      stepper motors. I am considering connecting 2 pc power supply's 12v
      output in series to get 24v. Is this a safe way to do this? Is
      there any hidden gotcha that I need to know?

      Also, can I connect pc power supplies in paralle to get higher
      current capacity?

      -thanks
      James
    • stan_eye_am
      ... drive ... 12v ... James, I only know a little about switch-mode supplies but I think it you can put them in series. Switch-mode supplies have some
      Message 2 of 2 , Mar 19, 2002
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        --- In sfrsa@y..., "urstest" <urstest@y...> wrote:
        > I have been using some old pc power supply for my projects. But
        > currently I am working on a new project that needs about 24v to
        drive
        > stepper motors. I am considering connecting 2 pc power supply's
        12v
        > output in series to get 24v. Is this a safe way to do this? Is
        > there any hidden gotcha that I need to know?
        >
        > Also, can I connect pc power supplies in paralle to get higher
        > current capacity?
        >
        > -thanks
        > James


        James,

        I only know a little about switch-mode supplies but I think it you
        can put them in series. Switch-mode supplies have some drawbacks -
        like they need a minimum current draw in order to work properly and
        they may only regulate the output voltage on one output of a multi-
        output supply (probably the 5V supply).

        Of course, if this works, all the outputs on the floating supply will
        be referenced to 12V so the 5V output will be at 17V, etc. Is this
        OK? You will want to verify that the ground of the floating supply
        (the one on top) is not connected to chassis or input ground. You can
        do this with a resistance meter when the supply is turned off. You
        should measure several tens of kilohms between the output ground and
        chassis ground. No R meter? Turn both on and try putting a large (say
        510 ohm) resistor from the output of one to the ground of the other.
        If it can be floated, the output will rise to the voltage you expect.
        If not, the R will get a bit hot!

        I would not recommend putting the outputs in parallel. This works
        with batteries becuase of the internal battery resistance. With
        regulated supplies, they will "fight" one another unless they are
        perfectly matched in output voltage (and they won't be). The higher
        voltage one will dump current into the lower voltage one in order to
        pull its output voltage up. Hence one will supply much more current
        than the other. A series R in the positive output leg of each placed
        before the supplies are joined will reduce this effect but will also
        reduce the regulation. The whole purpose of regulation is to make the
        supply output resistance appear to be zero!

        Perhaps others can fill in the blanks I have left.

        Good Luck,

        Stan
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